BIOL 271 | BIOL271 Module 5: Microbiology
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. Which component of the innate immune system is responsible for breaking down the
peptidoglycan layer of bacterial cell walls?
A. Lysozyme
B. Interferon
C. Complement C3b
D. Histamine
Correct Answer: A
Explanation: Lysozyme is a chemical barrier found in secretions like tears and saliva. It
specifically catalyzes the hydrolysis of the linkage between N-acetylglucosamine and N-
acetylmuramic acid in bacterial cell walls. This enzymatic action leads to the lysis of the
bacteria, particularly Gram-positive species. By destroying the structural integrity of the
cell wall, lysozyme prevents early bacterial colonization. It serves as a vital first-line
defense mechanism within the innate immune response.
2. During the process of inflammation, what is the term for the movement of white blood
cells through the intact walls of the capillaries into the surrounding tissue?
A. Opsonization
B. Diapedesis
C. Chemotaxis
D. Phagocytosis
Correct Answer: B
Explanation: Diapedesis involves the migration of leukocytes across the vascular
endothelium to reach the site of infection. This process is triggered by inflammatory signals
that increase vascular permeability. Once in the interstitial space, these cells follow
chemical gradients to find pathogens. This mechanism is essential for delivering
neutrophils and macrophages to damaged tissues. Without diapedesis, the immune system
could not effectively concentrate its cellular defenses at the point of injury.
3. Which antibody class is the first to be produced in response to a primary infection?
A. IgM
B. IgA
C. IgG
,D. IgE
Correct Answer: A
Explanation: IgM is a pentameric antibody that appears early in the primary immune
response. Its large size and multiple binding sites allow it to effectively agglutinate
pathogens and activate the complement system. It serves as a marker for recent exposure
to an antigen before the more specific IgG is produced. The high avidity of IgM compensates
for its lower affinity during the early stages of infection. This transition from IgM to other
classes is a hallmark of the adaptive immune maturation process.
4. MHC Class II molecules are primarily found on which of the following cell types?
A. Antigen-presenting cells (APCs)
B. Red blood cells
C. All nucleated cells
D. Cytotoxic T cells
Correct Answer: A
Explanation: MHC Class II molecules are expressed exclusively by professional antigen-
presenting cells like macrophages and B cells. These molecules present exogenous antigens
that have been internalized and processed. This presentation is crucial for the activation of
Helper T cells which coordinate the overall immune response. Unlike MHC Class I, which
signals internal infection, MHC Class II signals the presence of an external threat. This
distinction ensures that the immune system appropriately targets pathogens found in
extracellular environments.
5. Which type of immunity is acquired when a mother passes antibodies to her infant through
breast milk?
A. Natural active immunity
B. Natural passive immunity
C. Artificial active immunity
D. Artificial passive immunity
Correct Answer: B
Explanation: Natural passive immunity occurs when pre-formed antibodies are
transferred from one individual to another through biological processes. In the case of
breastfeeding, IgA antibodies are provided to the infant to protect against gastrointestinal
pathogens. This form of protection is immediate but temporary, as the infant’s body does
not produce its own memory cells. It provides a critical defense layer while the infant’s own
immune system is still developing. This natural transfer is a key component of neonatal
health and survival.
, 6. What is the primary function of Cytotoxic T lymphocytes (CTLs)?
A. Producing antibodies
B. Releasing histamine during allergy
C. Phagocytizing extracellular bacteria
D. Inducing apoptosis in infected host cells
Correct Answer: D
Explanation: Cytotoxic T cells recognize and bind to host cells displaying foreign antigens
on MHC Class I molecules. Once bound, they release perforins and granzymes that
penetrate the target cell membrane. This action triggers programmed cell death, or
apoptosis, to eliminate intracellular pathogens like viruses. By destroying the infected cell,
CTLs prevent the further replication and spread of the virus. This direct killing mechanism
is a cornerstone of the adaptive cellular immune response.
7. Which component of the complement system is responsible for the formation of the
Membrane Attack Complex (MAC)?
A. C1 to C4
B. C3a and C5a
C. C5b through C9
D. C3b only
Correct Answer: C
Explanation: The Membrane Attack Complex (MAC) is the end result of the complement
activation pathways. It is composed of the proteins C5b, C6, C7, C8, and multiple C9
molecules. The MAC assembles into a pore that inserts itself into the lipid bilayer of the
pathogen. This pore causes uncontrolled flow of water and ions, leading to osmotic lysis of
the cell. The ability to directly puncture bacterial membranes makes the MAC a powerful
tool in innate defense.
8. What is the role of opsonins in the immune response?
A. To coat pathogens and enhance phagocytosis
B. To inhibit viral replication
C. To trigger the release of histamine
D. To present antigens to T cells
Correct Answer: A
Explanation: Opsonization is the process where molecules like antibodies or complement
proteins coat the surface of a pathogen. This ‘tagging’ makes the pathogen much more
recognizable to phagocytic cells like neutrophils and macrophages. Phagocytes have
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. Which component of the innate immune system is responsible for breaking down the
peptidoglycan layer of bacterial cell walls?
A. Lysozyme
B. Interferon
C. Complement C3b
D. Histamine
Correct Answer: A
Explanation: Lysozyme is a chemical barrier found in secretions like tears and saliva. It
specifically catalyzes the hydrolysis of the linkage between N-acetylglucosamine and N-
acetylmuramic acid in bacterial cell walls. This enzymatic action leads to the lysis of the
bacteria, particularly Gram-positive species. By destroying the structural integrity of the
cell wall, lysozyme prevents early bacterial colonization. It serves as a vital first-line
defense mechanism within the innate immune response.
2. During the process of inflammation, what is the term for the movement of white blood
cells through the intact walls of the capillaries into the surrounding tissue?
A. Opsonization
B. Diapedesis
C. Chemotaxis
D. Phagocytosis
Correct Answer: B
Explanation: Diapedesis involves the migration of leukocytes across the vascular
endothelium to reach the site of infection. This process is triggered by inflammatory signals
that increase vascular permeability. Once in the interstitial space, these cells follow
chemical gradients to find pathogens. This mechanism is essential for delivering
neutrophils and macrophages to damaged tissues. Without diapedesis, the immune system
could not effectively concentrate its cellular defenses at the point of injury.
3. Which antibody class is the first to be produced in response to a primary infection?
A. IgM
B. IgA
C. IgG
,D. IgE
Correct Answer: A
Explanation: IgM is a pentameric antibody that appears early in the primary immune
response. Its large size and multiple binding sites allow it to effectively agglutinate
pathogens and activate the complement system. It serves as a marker for recent exposure
to an antigen before the more specific IgG is produced. The high avidity of IgM compensates
for its lower affinity during the early stages of infection. This transition from IgM to other
classes is a hallmark of the adaptive immune maturation process.
4. MHC Class II molecules are primarily found on which of the following cell types?
A. Antigen-presenting cells (APCs)
B. Red blood cells
C. All nucleated cells
D. Cytotoxic T cells
Correct Answer: A
Explanation: MHC Class II molecules are expressed exclusively by professional antigen-
presenting cells like macrophages and B cells. These molecules present exogenous antigens
that have been internalized and processed. This presentation is crucial for the activation of
Helper T cells which coordinate the overall immune response. Unlike MHC Class I, which
signals internal infection, MHC Class II signals the presence of an external threat. This
distinction ensures that the immune system appropriately targets pathogens found in
extracellular environments.
5. Which type of immunity is acquired when a mother passes antibodies to her infant through
breast milk?
A. Natural active immunity
B. Natural passive immunity
C. Artificial active immunity
D. Artificial passive immunity
Correct Answer: B
Explanation: Natural passive immunity occurs when pre-formed antibodies are
transferred from one individual to another through biological processes. In the case of
breastfeeding, IgA antibodies are provided to the infant to protect against gastrointestinal
pathogens. This form of protection is immediate but temporary, as the infant’s body does
not produce its own memory cells. It provides a critical defense layer while the infant’s own
immune system is still developing. This natural transfer is a key component of neonatal
health and survival.
, 6. What is the primary function of Cytotoxic T lymphocytes (CTLs)?
A. Producing antibodies
B. Releasing histamine during allergy
C. Phagocytizing extracellular bacteria
D. Inducing apoptosis in infected host cells
Correct Answer: D
Explanation: Cytotoxic T cells recognize and bind to host cells displaying foreign antigens
on MHC Class I molecules. Once bound, they release perforins and granzymes that
penetrate the target cell membrane. This action triggers programmed cell death, or
apoptosis, to eliminate intracellular pathogens like viruses. By destroying the infected cell,
CTLs prevent the further replication and spread of the virus. This direct killing mechanism
is a cornerstone of the adaptive cellular immune response.
7. Which component of the complement system is responsible for the formation of the
Membrane Attack Complex (MAC)?
A. C1 to C4
B. C3a and C5a
C. C5b through C9
D. C3b only
Correct Answer: C
Explanation: The Membrane Attack Complex (MAC) is the end result of the complement
activation pathways. It is composed of the proteins C5b, C6, C7, C8, and multiple C9
molecules. The MAC assembles into a pore that inserts itself into the lipid bilayer of the
pathogen. This pore causes uncontrolled flow of water and ions, leading to osmotic lysis of
the cell. The ability to directly puncture bacterial membranes makes the MAC a powerful
tool in innate defense.
8. What is the role of opsonins in the immune response?
A. To coat pathogens and enhance phagocytosis
B. To inhibit viral replication
C. To trigger the release of histamine
D. To present antigens to T cells
Correct Answer: A
Explanation: Opsonization is the process where molecules like antibodies or complement
proteins coat the surface of a pathogen. This ‘tagging’ makes the pathogen much more
recognizable to phagocytic cells like neutrophils and macrophages. Phagocytes have
